Lithium batteries have higher voltages and energy densities than conventional aqueous batteries that use an aqueous solution of a supporting salt as an electrolyte, and it is thus easy to make small, light-weight lithium batteries. Moreover, lithium batteries undergo less deterioration from, for example, self-discharging and have much higher long-term reliability than aqueous batteries. Lithium batteries therefore have been used in various applications, for example, as a main power source and a backup power source of small electronic devices.
Typical lithium batteries use metallic lithium or an alloy thereof as a negative electrode active material and a metal oxide such as manganese dioxide as a positive electrode active material and further contain an organic electrolyte. Generally, an organic electrolyte contains a non-aqueous solvent and a solute. For example, high-permittivity solvents such as propylene carbonate (PC) and γ-butyrolactone (GBL), low-boiling low-viscosity solvents such as 1,2-dimethoxyethane (DME) and 3-methyltetrahydrofuran (Me-THF), and similar solvents are used as such non-aqueous solvents. For example, lithium perchlorate (LiClO4), lithium tetrafluoroborate (LiBF4), and the like are used as such supporting salts.
However, lithium batteries that use manganese dioxide as a positive electrode active material are problematic in that a prolonged intermittent pulse discharge after high-temperature storage results in a rapidly increased internal resistance of the batteries and makes discharging impossible. Batteries for use as a main power source of small electronic devices and the like are required to have the ability to perform an intermittent large-current pulse discharge. Therefore, there is still room for improvement of manganese dioxide-containing lithium batteries used as a main power source of small electronic devices that are relatively often exposed to high temperatures.
Moreover, lithium batteries that use manganese dioxide as the positive electrode active material, once molded into coin-shaped batteries, result in the problem that gas may be generated after high-temperature storage at 100° C. or higher. No less than 90% of the gas generated inside the batteries is carbon dioxide, and it is therefore believed that the non-aqueous solvent contained in the organic electrolyte undergoes oxidative decomposition due mostly to the manganese dioxide serving as the positive electrode active material, and carbon dioxide is thus generated. In coin-shaped lithium batteries, the electrical connection between the positive electrode active material layer and the positive electrode current collector as well as the ionic conduction between the positive electrode, the separator, and the negative electrode are retained by the compression created by the sealing pressure.
Therefore, the pressure inside the battery that is created by the generation of gas in the battery and that is greater than the sealing pressure may inhibit a smooth electrical connection and ionic conduction, making a smooth discharge impossible even when there is battery capacity remaining. Therefore, there is a demand for a technique that inhibits the generation of gas during high-temperature storage.
Various positive electrode active materials for lithium batteries have been proposed. For example, λ-β type manganese dioxide having a specific surface area of 1 to 8 m2/g in which part of the manganese is substituted with another element so as to inhibit the reaction between manganese dioxide and an organic electrolyte have been proposed (for example, see Patent Document 1). In Patent Document 1, the element substituted for manganese is at least one element selected from the group consisting of nonmetallic elements of Groups 13 to 15, metalloid elements of Groups 13 to 15, alkali metals, alkaline earth metals, and metallic elements other than manganese.
In Patent Document 1, λ-β type manganese dioxides are produced according to a method including a baking step, an acid washing step, and a rebaking step.
In the baking step, manganese dioxide, a lithium salt, and a compound containing another element are baked at 550 to 950° C. for 5 to 20 hours to synthesize a spinel lithium manganese complex oxide.
In the acid washing step, acid washing removes lithium and the like from the spinel lithium manganese composite oxide. In this step, the lithium content of the spinel lithium manganese composite oxide is controlled to 0.2 to 1 mass %, and for this purpose it is necessary to use a strong acid having a pH of about 2, making the work environment very dangerous.
In the rebaking step, the required baking time is 2 to 10 hours.
As described above, with the technique disclosed in Patent Document 1, the steps of producing λ-β type manganese dioxide are complex, and the technique requires a long period of time, provides poor worker safety, and is very costly.
Manganese dioxide to which 0.1 to 2 wt % of boron and 0.02 to 2 wt % of phosphorus are added has been proposed to inhibit the dissolution of manganese in an organic electrolyte (for example, see Patent Document 2). The technique of Patent Document 2 is effective in preventing manganese from leaching into an organic electrolyte when the battery is stored at 70° C. However, when the battery is stored at 100° C. or higher, the generation of gas due to the decomposition of the organic electrolyte cannot be sufficiently inhibited.
A composite oxide of boron-containing lithium and manganese has been proposed to improve charge-discharge cycle characteristics (for example, see Patent Document 3). However, this composite oxide is a positive electrode active material for use in a secondary battery. This composite oxide is effective in improving the charge-discharge cycle characteristics and reducing the self-discharge rate of a secondary battery. However, when a battery is stored at 100° C. or higher, the generation of gas cannot be sufficiently inhibited. In addition, this composite oxide when having a large boron content functions to deteriorate the discharge capacity and the discharge voltage.
Lithium-containing manganese dioxide in which a specific lithium compound and a hydroxide, carbonate, or nitrate of a specific element are added to manganese dioxide and baked has been proposed to improve the discharge capacity of manganese dioxide (for example, see Patent Document 4). The technique of Patent Document 4 enhances the availability of lithium-containing manganese dioxide as a positive electrode active material by stabilizing its structure. However, when the battery is stored at 100° C. or higher, the generation of gas cannot be sufficiently inhibited by the technique of Patent Document 4 either.
Furthermore, addition of a sultone derivative such as 1,3-propanesultone to a mixed solvent of a carbonic acid ester and ether, for example, has been proposed as an improvement of an organic electrolyte (for example, see Patent Document 5). Addition of a sultone derivative to an organic electrolyte enhances the high-temperature storage characteristics of the organic electrolyte. However, in the organic electrolyte-containing lithium battery of Patent Document 5 also, the generation of gas cannot be sufficiently inhibited when the battery is stored at 100° C. or higher.
As described above, the lithium batteries of Patent Documents 1 to 5 do not have satisfactory high-temperature storage characteristics at 100° C. or higher.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2005-100944
Patent Document 2: Japanese Laid-Open Patent Publication No. 2003-217579
Patent Document 3: Japanese Laid-Open Patent Publication No. Hei 9-115515
Patent Document 4: Japanese Laid-Open Patent Publication No. Hei 9-139211
Patent Document 5: Japanese Laid-Open Patent Publication No. 2005-216867